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<H1 ALIGN="CENTER">Dynamically configured mass virtual hosting</H1>

<P>This document describes how to efficiently serve an arbitrary number
of virtual hosts with Apache 1.3. 

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Written by Tony Finch (fanf@demon.net) (dot@dotat.at).

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Some suggestions were made by Brian Behlendorf.

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<H2><A NAME="contents">Contents:</A></H2>

<UL>
<LI><A HREF="#motivation">Motivation</A>
<LI><A HREF="#overview">Overview</A>
<LI><A HREF="#simple">Simple dynamic virtual hosts</A>
<LI><A HREF="#homepages">A virtually hosted homepages system</A>
<LI><A HREF="#combinations">Using more than one virtual hosting system on the same server</A>
<LI><A HREF="#ipbased">More efficient IP-based virtual hosting</A>
<LI><A HREF="#oldversion">Using older versions of Apache</A>
<LI><A HREF="#simple.rewrite">Simple dynamic virtual hosts using <CODE>mod_rewrite</CODE></A>
<LI><A HREF="#homepages.rewrite">A homepages system using <CODE>mod_rewrite</CODE></A>
<LI><A HREF="#xtra-conf">Using a separate virtual host configuration file</A>
</UL>

<HR><H2><A NAME="motivation">Motivation</A></H2>

<P>The techniques described here are of interest if your
<CODE>httpd.conf</CODE> contains many
<CODE>&lt;VirtualHost&gt;</CODE> sections that are substantially the
same, for example:
<PRE>
NameVirtualHost 111.22.33.44
&lt;VirtualHost 111.22.33.44&gt;
	ServerName		           www.customer-1.com
	DocumentRoot		/www/hosts/www.customer-1.com/docs
	ScriptAlias  /cgi-bin/  /www/hosts/www.customer-1.com/cgi-bin
&lt;/VirtualHost&gt;
&lt;VirtualHost 111.22.33.44&gt;
	ServerName		           www.customer-2.com
	DocumentRoot		/www/hosts/www.customer-2.com/docs
	ScriptAlias  /cgi-bin/  /www/hosts/www.customer-2.com/cgi-bin
&lt;/VirtualHost&gt;
# blah blah blah
&lt;VirtualHost 111.22.33.44&gt;
	ServerName		           www.customer-N.com
	DocumentRoot		/www/hosts/www.customer-N.com/docs
	ScriptAlias  /cgi-bin/  /www/hosts/www.customer-N.com/cgi-bin
&lt;/VirtualHost&gt;
</PRE>
</P>

<P>The basic idea is to replace all of the static
<CODE>&lt;VirtualHost&gt;</CODE> configuration with a mechanism that
works it out dynamically. This has a number of advantages:
<OL>
    <LI>Your configuration file is smaller so Apache starts faster and
	uses less memory.
    <LI>Adding virtual hosts is simply a matter of creating the
	appropriate directories in the filesystem and entries in the DNS -
	you don't need to reconfigure or restart Apache.
</OL>
</P>

<P>The main disadvantage is that you cannot have a different log file
for each virtual host; however if you have very many virtual hosts
then doing this is dubious anyway because it eats file descriptors. It
is better to log to a pipe or a fifo and arrange for the process at
the other end to distribute the logs to the customers (it can also
accumulate statistics, etc.).</P>


<HR><H2><A NAME="overview">Overview</A></H2>

<P>A virtual host is defined by two pieces of information: its IP
address, and the contents of the <CODE>Host:</CODE> header in the HTTP
request. The dynamic mass virtual hosting technique is based on
automatically inserting this information into the pathname of the file
that is used to satisfy the request. This is done most easily using
<A HREF="../mod/mod_vhost_alias.html"><CODE>mod_vhost_alias</CODE></A>,
but if you are using a version of Apache up to 1.3.6 then you must use
<A HREF="../mod/mod_rewrite.html"><CODE>mod_rewrite</CODE></A>. Both
of these modules are disabled by default; you must enable one of them
when configuring and building Apache if you want to use this technique.</P>

<P>A couple of things need to be `faked' to make the dynamic virtual
host look like a normal one. The most important is the server name
which is used by Apache to generate self-referential URLs, etc. It
is configured with the <CODE>ServerName</CODE> directive, and it is
available to CGIs via the <CODE>SERVER_NAME</CODE> environment
variable. The actual value used at run time is controlled by the
<A HREF="../mod/core.html#usecanonicalname"><CODE>UseCanonicalName</CODE></A>
setting. With <CODE>UseCanonicalName Off</CODE> the server name
comes from the contents of the <CODE>Host:</CODE> header in the
request. With <CODE>UseCanonicalName DNS</CODE> it comes from a
reverse DNS lookup of the virtual host's IP address. The former
setting is used for name-based dynamic virtual hosting, and the latter
is used for IP-based hosting. If Apache cannot work out the server
name because there is no <CODE>Host:</CODE> header or the DNS lookup
fails then the value configured with <CODE>ServerName</CODE> is used
instead.</P>

<P>The other thing to `fake' is the document root (configured
with <CODE>DocumentRoot</CODE> and available to CGIs via the
<CODE>DOCUMENT_ROOT</CODE> environment variable). This setting
is used by the core module when mapping URIs to filenames, but
when the server is configured to do dynamic virtual hosting that
job is taken over by another module. If any CGIs or SSI documents
make use of the <CODE>DOCUMENT_ROOT</CODE> environment variable
they will therefore get a misleading value; there isn't any way to
change <CODE>DOCUMENT_ROOT</CODE> dynamically.</P>


<HR><H2><A NAME="simple">Simple dynamic virtual hosts</A></H2>

<P>This extract from <CODE>httpd.conf</CODE> implements the virtual
host arrangement outlined in the <A HREF="#motivation">Motivation</A>
section above, but in a generic fashion using
<CODE>mod_vhost_alias</CODE>.</P>

<PRE>
# get the server name from the Host: header
UseCanonicalName Off

# this log format can be split per-virtual-host based on the first field
LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon
CustomLog logs/access_log vcommon

# include the server name in the filenames used to satisfy requests
VirtualDocumentRoot /www/hosts/%0/docs
VirtualScriptAlias  /www/hosts/%0/cgi-bin
</PRE>

<P>This configuration can be changed into an IP-based virtual hosting
solution by just turning <CODE>UseCanonicalName Off</CODE> into
<CODE>UseCanonicalName DNS</CODE>. The server name that is inserted
into the filename is then derived from the IP address of the virtual
host.</P>


<HR><H2><A NAME="homepages">A virtually hosted homepages system</A></H2>

<P>This is an adjustment of the above system tailored for an ISP's
homepages server. Using a slightly more complicated configuration we
can select substrings of the server name to use in the filename so
that e.g. the documents for <SAMP>www.user.isp.com</SAMP> are found in
<CODE>/home/user/</CODE>. It uses a single <CODE>cgi-bin</CODE>
directory instead of one per virtual host.</P>

<PRE>
# all the preliminary stuff is the same as above, then

# include part of the server name in the filenames
VirtualDocumentRoot /www/hosts/%2/docs

# single cgi-bin directory
ScriptAlias  /cgi-bin/  /www/std-cgi/
</PRE>

<P>There are examples of more complicated
<CODE>VirtualDocumentRoot</CODE> settings in
<A HREF="../mod/mod_vhost_alias.html">the
<CODE>mod_vhost_alias</CODE> documentation</A>.</P>


<HR><H2><A NAME="combinations">Using more than one virtual hosting
system on the same server</A></H2>

<P>With more complicated setups you can use Apache's normal
<CODE>&lt;VirtualHost&gt;</CODE> directives to control the scope of
the various virtual hosting configurations. For example, you could
have one IP address for homepages customers and another for commercial
customers with the following setup. This can of course be combined
with conventional <CODE>&lt;VirtualHost&gt;</CODE> configuration
sections.</P>

<PRE>
UseCanonicalName Off

LogFormat "%V %h %l %u %t \"%r\" %s %b" vcommon

&lt;Directory /www/commercial&gt;
	Options FollowSymLinks
	AllowOverride All
&lt;/Directory&gt;

&lt;Directory /www/homepages&gt;
	Options FollowSymLinks
	AllowOverride None
&lt;/Directory&gt;

&lt;VirtualHost 111.22.33.44&gt;
	ServerName www.commercial.isp.com

	CustomLog logs/access_log.commercial vcommon

	VirtualDocumentRoot /www/commercial/%0/docs
	VirtualScriptAlias  /www/commercial/%0/cgi-bin
&lt;/VirtualHost&gt;

&lt;VirtualHost 111.22.33.45&gt;
	ServerName www.homepages.isp.com

	CustomLog logs/access_log.homepages vcommon

	VirtualDocumentRoot /www/homepages/%0/docs
	ScriptAlias         /cgi-bin/ /www/std-cgi/
&lt;/VirtualHost&gt;
</PRE>


<HR><H2><A NAME="ipbased">More efficient IP-based virtual hosting</A></H2>

<P>After <A HREF="#simple">the first example</A> I noted that it is
easy to turn it into an IP-based virtual hosting setup. Unfortunately
that configuration is not very efficient because it requires a DNS
lookup for every request. This can be avoided by laying out the
filesystem according to the IP addresses themselves rather than the
corresponding names and changing the logging similarly. Apache will
then usually not need to work out the server name and so incur a DNS
lookup.</P>

<PRE>
# get the server name from the reverse DNS of the IP address
UseCanonicalName DNS

# include the IP address in the logs so they may be split
LogFormat "%A %h %l %u %t \"%r\" %s %b" vcommon
CustomLog logs/access_log vcommon

# include the IP address in the filenames
VirtualDocumentRootIP /www/hosts/%0/docs
VirtualScriptAliasIP  /www/hosts/%0/cgi-bin
</PRE>


<HR><H2><A NAME="oldversion">Using older versions of Apache</A></H2>

<P>The examples above rely on <CODE>mod_vhost_alias</CODE> which
appeared after version 1.3.6. If you are using a version of Apache
without <CODE>mod_vhost_alias</CODE> then you can implement this
technique with <CODE>mod_rewrite</CODE> as illustrated below, but
only for Host:-header-based virtual hosts.</P>

<P>In addition there are some things to beware of with logging. Apache
1.3.6 is the first version to include the <CODE>%V</CODE> log format
directive; in versions 1.3.0 - 1.3.3 the <CODE>%v</CODE> option did
what <CODE>%V</CODE> does; version 1.3.4 has no equivalent. In
all these versions of Apache the <CODE>UseCanonicalName</CODE>
directive can appear in <CODE>.htaccess</CODE> files which means that
customers can cause the wrong thing to be logged. Therefore the best
thing to do is use the <CODE>%{Host}i</CODE> directive which logs the
<CODE>Host:</CODE> header directly; note that this may include
<CODE>:port</CODE> on the end which is not the case for
<CODE>%V</CODE>.</P>


<HR><H2><A NAME="simple.rewrite">Simple dynamic virtual hosts
using <CODE>mod_rewrite</CODE></A></H2>

<P>This extract from <CODE>httpd.conf</CODE> does the same thing as
<A HREF="#simple">the first example</A>. The first half is very
similar to the corresponding part above but with some changes for
backward compatibility and to make the <CODE>mod_rewrite</CODE> part
work properly; the second half configures <CODE>mod_rewrite</CODE> to
do the actual work.</P>

<P>There are a couple of especially tricky bits: By default,
<CODE>mod_rewrite</CODE> runs before the other URI translation modules
(<CODE>mod_alias</CODE> etc.) so if they are used then
<CODE>mod_rewrite</CODE> must be configured to accommodate them.
Also, mome magic must be performed to do a per-dynamic-virtual-host
equivalent of <CODE>ScriptAlias</CODE>.</P>

<PRE>
# get the server name from the Host: header
UseCanonicalName Off

# splittable logs
LogFormat "%{Host}i %h %l %u %t \"%r\" %s %b" vcommon
CustomLog logs/access_log vcommon

&lt;Directory /www/hosts&gt;
	# ExecCGI is needed here because we can't force
	# CGI execution in the way that ScriptAlias does
	Options FollowSymLinks ExecCGI
&lt;/Directory&gt;

# now for the hard bit

RewriteEngine On

# a ServerName derived from a Host: header may be any case at all
RewriteMap  lowercase  int:tolower

## deal with normal documents first:
# allow Alias /icons/ to work - repeat for other aliases
RewriteCond  %{REQUEST_URI}  !^/icons/
# allow CGIs to work
RewriteCond  %{REQUEST_URI}  !^/cgi-bin/
# do the magic
RewriteRule  ^/(.*)$  /www/hosts/${lowercase:%{SERVER_NAME}}/docs/$1

## and now deal with CGIs - we have to force a MIME type
RewriteCond  %{REQUEST_URI}  ^/cgi-bin/
RewriteRule  ^/(.*)$  /www/hosts/${lowercase:%{SERVER_NAME}}/cgi-bin/$1  [T=application/x-httpd-cgi]

# that's it!
</PRE>


<HR><H2><A NAME="homepages.rewrite">A homepages system
using <CODE>mod_rewrite</CODE></A></H2>

<P>This does the same thing as <A HREF="#homepages">the
second example</A>.</P>

<PRE>
RewriteEngine on

RewriteMap   lowercase  int:tolower

# allow CGIs to work
RewriteCond  %{REQUEST_URI}  !^/cgi-bin/

# check the hostname is right so that the RewriteRule works
RewriteCond  ${lowercase:%{SERVER_NAME}}  ^www\.[a-z-]+\.isp\.com$

# concatenate the virtual host name onto the start of the URI
# the [C] means do the next rewrite on the result of this one
RewriteRule  ^(.+)  ${lowercase:%{SERVER_NAME}}$1  [C]

# now create the real file name
RewriteRule  ^www\.([a-z-]+)\.isp\.com/(.*) /home/$1/$2

# define the global CGI directory
ScriptAlias  /cgi-bin/  /www/std-cgi/
</PRE>


<HR><H2><A NAME="xtra-conf">Using a separate virtual host configuration file</A></H2>

<P>This arrangement uses more advanced <CODE>mod_rewrite</CODE>
features to get the translation from virtual host to document root
from a separate configuration file. This provides more flexibility but
requires more complicated configuration.</P>

<P>The <CODE>vhost.map</CODE> file contains something like this:
<PRE>
www.customer-1.com  /www/customers/1
www.customer-2.com  /www/customers/2
# ...
www.customer-N.com  /www/customers/N
</PRE>
</P>

<P>The <CODE>http.conf</CODE> contains this:
<PRE>
RewriteEngine on

RewriteMap   lowercase  int:tolower

# define the map file
RewriteMap   vhost      txt:/www/conf/vhost.map

# deal with aliases as above
RewriteCond  %{REQUEST_URI}               !^/icons/
RewriteCond  %{REQUEST_URI}               !^/cgi-bin/
RewriteCond  ${lowercase:%{SERVER_NAME}}  ^(.+)$
# this does the file-based remap
RewriteCond  ${vhost:%1}                  ^(/.*)$
RewriteRule  ^/(.*)$                      %1/docs/$1

RewriteCond  %{REQUEST_URI}               ^/cgi-bin/
RewriteCond  ${lowercase:%{SERVER_NAME}}  ^(.+)$
RewriteCond  ${vhost:%1}                  ^(/.*)$
RewriteRule  ^/(.*)$                      %1/cgi-bin/$1
</PRE>
</P>

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